Document limitations

[eskimor]

Related to #823 it would be really good to have it clearly documented right in the README what the limitations of the tools are. E.g. what is mocked/what is real? On what can you rely on what not? What are things that could work with chopsticks, but still fail in production?

[xlc]

Generally, anything that involves something more than onchain STF new_state = f(old_state) are not guaranteed to work.
In practice, here is an incomplete list that I can think of:

• mocked tx pool
• no real block finalization
• mocked inherents
• simulated XCM channels